1. Field of the Invention
The present invention relates to a liquid ejection head that supplies energy to ejection energy generating elements to provide the energy to the liquid to eject the liquid through ejection ports.
2. Description of the Related Art
In many ink jet printing apparatuses commonly used, a print head as a liquid ejection head has been formed by laminating an orifice plate to a substrate with liquid supply ports and the like formed therein, from the past. The structure of such a print head is shown in FIGS. 11A and 11B. FIG. 11A shows a plan view of a conventional liquid ejection head 501. FIG. 11B shows a sectional view of the conventional liquid ejection head 501 taken along line XIB-XIB in FIG. 11A. In this form of print head, the substrate 503 and the orifice plate 502 are laminated together to form a common liquid chamber 504 in a part of the space between the substrate 503 and the orifice plate 502. A liquid supply port 505 is formed through the substrate 503 so as to communicate with the common liquid chamber 504. Liquid channels 507 extend in communication with the common liquid chamber 504. Pressure chambers 508 are each formed at a portion which is opposite the common liquid chamber 504 in the liquid channels 507. Ejection ports 506 are each formed in the orifice plate 502 so as to communicate with a corresponding one of the pressure chambers 508. Heaters 509 are each located at a position corresponding to one of the ejection ports 506 and serves as an ejection energy generating element that supplies ejection energy to a liquid in the pressure chamber 508. The liquid supplied to the common liquid chamber 504 via the liquid supply port 505 is fed to the pressure chamber 508 via the liquid channels 507. In the pressure chamber 508, the liquid is supplied with energy by the heaters 509 and thus ejected through the ejection ports 506.
In the print head 501, shown in FIGS. 11A and 11B, the liquid is fed in only one direction, from the liquid supply port 505 to the ejection ports 506.
When such a print head 501 is used to eject the liquid for printing, bubbles generated by the heaters 509 grow disproportionately from the pressure chamber 508 toward the liquid supply port 505. Thus, the liquid is ejected while being subjected to a force in this direction. At this time, a trailing part of the ejected liquid is pulled toward the common chamber 504 and torn off. Consequently, these trailing parts, called satellites, are inappropriately small and are prone to become mist floating inside a printer housing instead of impacting a print medium.
Even if the satellites impact the print medium instead of becoming the floating mist, the satellites, having a small mass, are readily affected by air currents; a direction in which the satellites fly is prone to be varied by the air currents. As a result, a position on the print medium at which each satellite impacts the print medium varies, resulting in the high likelihood of density unevenness.
When the satellites are ejected under a force acting toward the common liquid chamber 504, the direction in which the satellites fly is different from a direction in which main droplets fly. Thus, when the print head prints the print medium while performing scan, the manner in which the main droplets and the satellites overlap varies between a forward travel and a backward travel. Images obtained by printing are thus prone to suffer density unevenness.
Measures against the inappropriately small satellite portion are disclosed in Japanese Patent Laid-Open No. 60-206653 (1985) and U.S. Pat. No. 6,660,175. FIG. 12A is a perspective view showing a print head disclosed in Japanese Patent Laid-Open No. 60-206653 (1985); in FIG. 12A, the print head is disassembled into components. FIG. 12B is a sectional view of the periphery of an ejection port that is an essential part of the print head with the assembled components. FIG. 13A is an enlarged broken sectional view of an essential part of a print head disclosed in U.S. Pat. No. 6,660,175. FIG. 13B is a sectional view of a print head shown in FIG. 13A.
In Japanese Patent Laid-Open No. 60-206653 (1985) and U.S. Pat. No. 6,660,175, described above, ink guided to an ink supply port is further guided in an ejection direction. The ink is then guided in a direction orthogonal to the ejection direction. The ink is then provided with heat energy by heaters. Passages through which the ink is fed to ejection ports are formed in a direction from opposite sides of the ejection ports toward the ejection ports. Since the ink to be ejected is fed from the opposite sides of the ejection ports to the ejection ports, a possible one-sided ink flow is inhibited which may affect the growth of bubbles when the ink is ejected. This inhibits a one-sided force from being applied to the ink to be ejected.
Consequently, the bubbles grow and shrink substantially symmetrically with respect to the heater. Thus, the trailing of the ejected ink is prone to be straight, short, and thick. As a result, satellites formed by breakage of the trailing during the process of formation of droplets are prone to be large. In connection with the direction in which the droplets fly, the droplets are ejected exactly along the ejection direction almost orthogonal to an ejection port forming surface. Further, the direction in which the main droplets of the ejected ink fly is exactly along the ejection direction almost orthogonal to an ejection port forming surface, too. Thus, large satellites are generated when the liquid is ejected. Accordingly, the position at which each satellite impacts is unlikely to be affected by air currents, thus stabilizing the ejection direction of the droplets. Therefore, even if printing is performed at a high speed or with small droplets, density unevenness is unlikely to occur. Furthermore, the larger satellites increase a rate at which the satellites reach the print medium, reducing the mist floating in the printer housing instead of impacting the print medium. This reduces the possible contamination of the interior of the printer main body or sheet surfaces caused by the attachment of the floating mist. This makes an electric substrate and an encoder unlikely to become defective. When the satellites and the main droplets are ejected exactly along the direction orthogonal to the ejection port forming surface, it is possible to reduce the variation in the impacting positions of each main droplet and the corresponding satellite between the forward travel and backward travel of a printing operation. Consequently, the density variation is unlikely to occur during the reciprocating printing operation. As a result, density unevenness is unlikely to occur in images obtained on the print medium.
However, according to Japanese Patent Laid-Open No. 60-206653 (1985), the ink stored in a reservoir is further guided in the ink ejection direction through supply pipes. However, the guided ink communicates only with the vicinity of a central portion of ink channels in the direction of the row of ejection ports. Thus, the ink in the common liquid chamber which is positioned in the central portion of the line of ink channels is ejected or sucked by suction recovery. Consequently, the ink stored in the central portion of the ink channels is discharged from the print head instead of remaining in the common liquid chamber. However, the ink in the common liquid chamber which is positioned away from the supply pipes is unlikely to flow even with suction recovery. Thus, the ink stored in this site is prone to remain instead being sucked. Consequently, bubbles are prone to remain in the site and may affect ink ejection. Ejection characteristics are thus likely to vary. This makes it difficult to maintain the appropriate ejection condition of the print head and to stabilize ejection.
According to the method in U.S. Pat. No. 6,660,175, holes are formed in a layer located between the substrate and the orifice plate in the print head. Ink guided to the ink supply port is guided in the ejection direction through the holes. However, since the ink is fed to each of the ejection ports through the corresponding hole, when the ink passes through the hole, the channel forming the hole offers resistance to the ink flow. If the size of the hole is smaller, the resistance increases more. If the length of the channel in the hole is longer, the resistance increases more. The higher resistance reduces a speed at which new ink is refilled (hereinafter referred to as a refill speed) as well as a frequency at which droplets are repeatedly ejected (hereinafter referred to as a driving frequency). This reduces the throughput of a printing apparatus using this print head.
Possible measures against the resistance to the ink flow are to increase the diameter of each of the holes and to reduce the thickness of the layer in which the holes are formed. One of the measures, the increase in the diameter of the hole, enables a reduction in the resistance to the ink flow when the ink passes through the hole. This improves the throughput of the printing apparatus. However, the increased diameter of the hole increases the size of each of the pressure chambers and thus the distance between each ejection port and thus between each ejected droplet. This reduces the density of the ejection ports on the print head and thus a resolution provided by the ejected droplets. The reduced resolution finally reduces the throughput of the printing apparatus. In connection with the measure of reducing the thickness of the layer in which the holes are formed to reduce the length of the channel formed by each hole, an extreme reduction in the thickness of the layer prevents the required strength of the print head from being maintained. Furthermore, the reduced thickness of the layer reduces the amount of heat externally diffused via the layer and thus the amount of heat radiated. Thus, heat generated by the heater cannot be sufficiently released. Consequently, the temperature of the heater portion increases significantly. This prevents the driving frequency from being increased in order to inhibit a rise in the temperature of the print head. Therefore, also in this case, the throughput cannot be improved.